US2266981A - Inhibiting formation of natural gas hydrates - Google Patents

Description

Dec. 23; 1941. B. MILLER 6 INHIBITING FORMATION OF NATURAL GAS H YDRATES' Filed April 25, 1936 PKFSSUIfE raw MA /r LI um LEVEL A 221245 aux; 24

@115 25g I [fa/FEED REULATaf 643 LINE INVENTOR I .BENJ/IMM/ MILLER BY ATTORNEY than the freezing temperature of water.

Patented Dec. 23, 1941 ArsNr OFFICE INHIBITIN G FORMATION OF NATURAL GAS HYDRATES Benjamin Miller, Richmond Hill, N. Y., assignor to Cities Service Oil Company, New York, N. Y., a corporation of Pennsylvania Application April 25, 1936, Serial No. 76,486

11 Claims. (Cl. 48-190) This invention relates to a process for inhibiting formation of solid hydrates in high pressure natural gas transmission lines during cold weather.

Natural gas transmission pipe lines are usually buried in the ground below the frost line so that difdculty is seldomencountered from formation of ice obstructions in the line during cold weather. Nevertheless the gas carrying capacity of such lines is often seriously reduced during cold weather owing to the formation of deposits of solid hydrates of the normally gaseous hydrocarbon components of the gas. At the high pressures which are usually maintained in such transmission lines formation of solid hydrates 15 may occur at temperatures several degrees higher The highest temperature at which solid gas hydrates can exist in any particular transmission line depends on the pressure of the gas, the composition of the gas, and the concentration of water vapor in the gas. The exact composition of the hydrates is difflcult to determine and of course varies as the composition of the gas varies.

The principal part of the solid deposits is probably the hexahydrate of methane.

The primary object of the present invention is to provide an economically practicable and eiflcient method for inhibiting formation of solid gas hydrate in natural gas transmission lines during cold weather. Since one of the principal factors governing formation of solid gas hydrate is that of pressure, one possiblevmethod of preventing their formation would bto operate the line at a presproblem, however, because the transmission line must normally be operated at a higher pressure than the hydrate decomposition pressure-in order to carry the volume of gas required by its rated capacity. An alternative method would be to reduce the partial pressure of water vapor in the gas entering the line below the water vapor pressure of the hydrates normally formed at the minimum operating temperature and maximum operating pressure of the line. This method,

while thoroughly practical, requires a large capital investment which cannot always be justified.

Essentially the process forming the subject of the present invention contemplates inhibiting formation of gaseous hydrocarbon hydrates in a densation from the gas at the points of minimum temperature obtaining in the line of a liquid water solution. This result is effected by adding to the gas flowing through the line at a point 5 on the up-stream side of the point of minimum temperature, one or more of a group of volatile and water soluble fluids in amount sumcient to form a condensate with water which does not combine with the gaseous hydrocarbons to form solid hydrates at the mini-mum temperature and maximum operating pressure of the line. Among the fluids which are suitable for this purpose, monohydric aliphatic alcohols in the range from methanol through amyl alcohol are preferred because they may be added without injurious effects on materials which the gas may contact;

although the corresponding aliphatic ketones and acids and certain gases including carbon dioxide and ammonia may also be used. Secondary butyl alcohol is a preferred reagent, because it has a vapor pressure not greatly different from that of water. However, from a practical and economical standpoint methanol at present seems to be the best reagent of the group indicated, with ethanol as a probable second choice.- preferred plan of operation is to continuously introduce into the gas line during Fall and Winter a small measured proportion of the volatile water soluble fluid reagent in amount suflicient to prevent solid hydrate formation.

With the above and other objects and features in view, the invention'will be hereinafter more particularly described by reference to the accompanying diagrammatic illustration of a preferred type of equipment for introducing the fluid reagent into the natural gas transmission line.

Referring to the drawing, Ill indicates a small a broken section of a natural gas transmission line located at a point on the high pressureor upstream side of any point in the line where deposition of gas hydrates might occur. I2 indicates a supply tank for a volatile water solublefluid reagent to be used for inhibiting formation of natural gas hydrates in line l0. Injection of the fluid reagent into the line would normally 1 be made at the compressor station, downstream from the aftercoolers, preferably downstream from the first drip. In the case of lines having several compressor stations, injection might be made at each station, or at alternate stations For example, tankl! may berconsidercd as a source of supply for methanol. I4 is a combined pressure-gravity feed tank to which the methanol natural gas transmission line by effecting conis conducted by a valved pipe l6 from tank l2.

The'

Tank I4 is provided with a gauge glass Hi to enable the operator to observe the level of methanol in tank l4. Methanol is fed (preferably at a continuous controlled rate) from tank l4 into line I through a pipe equipped with a check valve 22. On the high pressure (upstream) side of the point of connection of feed proportion the rate of feed of methanol to line H1 in accordance with the metered rate of flow of natural gas through line l0.

. When a volatile water' soluble fluid reagent such as methanol is added to the natural gas flowing through line It at a rate not substantially in excess of that necessary to inhibit hydrate formation at a more or less remote point in the line, under normal conditions of temperature obtaining in the line the added alcohol may all be vaporized.v As the temperature drops further along the line, condensation of a solution of water in alcohol will occur. The water reduces the alcohol vapor pressure, and the alcohol reduces the water vapor pressure, so that more water is condensed than if the alcohol were not present.

The quantity of alcohol in the condensate is proportional to the quantity of water in the condensate.

Knowing the minimum temperature occurring in the line and the maximum pressure at which it is necessary to operate the line, the amount of alcohol which must be added to a definite volume of natural gas flowing through the line can be determined if we also know the initial saturation temperature of the water in the gas, and the critical pressure at which the solid hydrate decomposes at the point of minimum temperature. Of course this critical pressure at. which the hydrate decomposes depends on the composition of the hydrate, which depends in turn on the composition of the gas.

Assuming that'investigation has shown that with a particular gas, solid hydrate may exist in a transmission line at a point in the line where the temperature is 36 F. when the pressure ex-. ceeds 300 lbs. per square inch absolute, and that the initial saturation temperataure of the water in the gas is 60 F., then the amount of ethanol which must be added to each million cubic feet of gas passing through the line in order to effectively inhibit solid hydrate formation when operating the line at a desired maximum pressure of 450 lbs. per square inch can be determined as approximately 7.5 lbs., and the amount of ethanol required for the same purpose with an operating pressure of 800 lbs. per square inch would be about 10.5 lbs. If methanol were substituted for ethanol, the amount required to inhibit hydrate formation in the same gas at the same temperature would be about 10 lbs. at 450 lbs. pressure, and 13.5 lbs. at 800 lbs. maximum pressure.

Minimum temperatures below 32 F. are seldom encountered in natural gas transmission lines, and such lines are, not normally operated at pressures higher than 00 lbs. per square inch. Moreover at thistemprature the natural gas which is handld by such lines does not usually form solid hydrate at pressures below about 300 lbs. per square inch. Consequently any monohydric aliphatic alcohol with from 1 to 5 carbon atoms -in its molecule will satisfactorily inhibit hydrate formation in such lines when added thereto in proportions of 2-10 lbs. of dry alcohol per million cubic feet of gas carried by the line.

Having thus described the invention, what is claimed as new is:

1. The method of inhibiting formation of solid aliphatic hydrocarbon hydrates in high pressure natural gas transmission pipe lines at temperatures above the freezing point of water, which comprises adding to the gas a volatile water soluble fluid which is without injurious effects upon the materials of the pipe line and in amounts sufiicient to lower the water vapor concentration in the gas below the critical value for hydrate formation at the lowest temperature and highest pressure obtaining in the line.

2. The method of inhibiting formation of solid hydrates of normally gaseous aliphatic hydrocarbons in high pressure natural gas transmission pipe lines at temperatures above .the freezing point of water, which comprises continuously adding to the gas a small stream of a volatile water soluble fluid which is without injurious effects upon the materials ofthe pipe line and at a rate proportioned in accordance with thevolume of the gas so as to lower the water vapor concentration in the gas below the critical value for hydrate formation at the lowest temperature and highest pressure obtaining in the line.

3. The method for inhibiting formation of methane hydrate in high pressure natural gas transmission pipe lines at temperatures above the freezing point of water, which comprises continuously adding to the gas during periods of cold weather a monohydric aliphatic alcohol having from one to five carbon atoms in its molecule at the rate of approximately two to ten lbs. of alcohol to each million cubic feet of gas flowing through the line. D

4. The method for inhibiting formation of methane hydrate in high pressure natural gas I high pressure natural gas transmission lines by the formation of solid hydrocarbon hydrates in which pressures upward of 300 lbs. per square inch are used, which comprises introducing a relatively small proportion of a volatile water soluble fluid which is without injurious effects upon the materials of the pipe line into the gas in the line during periodsof cold weather when the gas in the line is at temperatures above the freezing point of water which would normally cause the formation of hydrocarbon hydrates in the line, said water soluble fluid being adapted to prevent the formation of said hydrates.

6. The method of preventing the stoppage of high pressure natural gas transmission lines by the formation of solid hydrocarbon hydrates therein from the hydrocarbons and the moisture present in the lines, which comprises introducing a relatively small proportion of a volatile water soluble fluid into the gas flowing through the-line during periods of cold weather when the gas temperatures are above the freezing point of water but sufliciently low to normally cause the formation of solid hydrocarbon hydrates, said watersoluble fluid introduced into the line being selected from the group of fluids consisting of the aliphatic monohydric alcohols,

ketones and acids containing from one to flve I carbon atoms inclusive in the molecule.

'7. The method of inhibiting the formation of solid hydrates of normally gaseous aliphatic hydrocarbons in high pressure natural gas transmission pipe lines from moisture contained in the gas, passing therethrough, which comprises effecting condensation of a liquid water solution from the gas in the pipe line at gas temperatures above the freezing point of water, said condensation being efiected by continuously introducing a volatile water soluble fluid which is without injurious effects upon the materials of the pipe line into the line at a point on the upstream side of the line section of minimum temperature 'so that a water solution of the introduced fluid is formed, said fluid being introduced at a rate proportioned in accordance with the volume of gas flowing through the line.

8. The method of preventing the formation of deposits of solid hydrocarbon hydrates in high pressure natural gas transmission lines normally caused by the formation of solid hydrocarbon injected into the gas flowing in the line in sufficient proportion to prevent the formation of deposits of solid hydrocarbon hydrates in the line.

9. The method of preventing the formation of deposits of solid hydrocarbon hydrates in high pressure natural gas transmission lines normally caused by the formation of solid hydrocarbon hydrates therein from the hydrocarbons and moisture contained in the gas being transmitted, which comprises introducing a relatively small proportion of a monohydric aliphatic alcohol having from one to five carbon atoms in its molecule into the gas flowing in the transmission line when the gas temperature is above the freezing point of water but is at temperature and pressureconditions which would normally cause the formation of solid hydrocarbon hydrates in the line, said alcohol being introduced into the gas flowing in the line in suflicient proportion to prevent formation of deposits of solid hydrocarbon hydrates therein.

10. The method of preventing the formation of deposits of solid hydrocarbon hydrates in gas transmission lines as defined by claim 9 in which said alcohol is methanol.

ll. The method of inhibiting formation of solid hydrates of normally gaseous aliphatic hydrocarbons in natural gas transmission lines operated at a pressure of approximately 300 lbs.

per square inch and above at temperatures above the freezing point-of water, which comprises continuously adding secondary butyl alcohol to the gas during periods of cold weather at the rate of approximately two to ten pounds of alcohol to each million .cubic feet of gas flowing through the line.

BENJAMIN MILLER.

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